1. Field of the Invention
This invention relates to an oriented polyethylene film with an excellent moistureproofness and optical property, comprising a composition of high density polyethylene and petroleum resins or linear low density polyethylene and having a specified crosslinking structure in the thickness structure of the film.
2. Description of the Prior Art
Generally, the property of a moistureproof wrapping material has been classified into four grades as to water vapor transmission rate, i.e. less than 1 g/m.sup.2.24 hr, 2.5 g/m.sup.2.24 hr, 5 g/m.sup.2.24 hr and more than 5 g/m.sup.2.24 hr. The class having a water vapor transmission rate of 5 g/m.sup.2.24 hr corresponds to the so-called biaxially oriented polypropylene film (OPP film) and the class having that of 2.5 g/m.sup.2.24 hr corresponds to the so-called KOP film consisting of OPP film coated with polyvinylidene chloride. For the class having a water vapor transmission rate of less than 1 g/m.sup.2.24 hr, it is required to subject the OPP film to vapor deposition with aluminum or to lamination with an aluminum foil.
On the other hand, high density polyethylene films having superior mechanical strength, cold resistance, edge opening and slippage have been applied to various uses including wrapping or packaging materials, but have not been used in the field where seeing-through is required because of being inferior in optical properties such as clarity and lustre, compared to polypropylene, low density polyethylene, polyvinyl chloride, etc. Therefore, it has been proposed to improve the clarity of polyethylene film by uniformly crosslinking by radiation and then biaxially stretching the film. However, the crosslinked and oriented polyethylene film obtained by this method is improved as to its clarity and strength, but does not have sufficient moistureproofness.
Thus, the inventors have proposed a polyethylene film which is crosslinked and oriented in such a manner that the degree of crosslinking inwardly decreases across the thickness of the film, and a process for producing the same, thus obtaining a crosslinked and oriented polyethylene film excellent in optical properties such as clarity and lustre and moistureproofness, as disclosed in Japanese Patent Laid-Open Publication Nos. 174321/1984 and 174322/1984.
However, this crosslinked and oriented polyethylene film has a more excellent moistureproofness than the OPP film, but the moistureproofness is not necessarily sufficient as compared with polyvinylidene chloride film.
Up to the present time, polyvinyl chloride films, oriented polystyrene films, foamed polystyrene films, etc. have been used as a film for a shrink label of a oneway can or glass bottle. However, these films experience a problem of retort ability because of occurrence of deteriorations such as becoming hazy and splitting during retort sterilization, although these films are excellent in clarity, printability, heat shrinkage property, etc.
On the other hand, various polyolefin films for shrink labels are known which consist of linear low density polyethylene, a composition comprising linear low density polyethylene and polypropylene and a composition comprising polypropylene and a petroleum resin. However, these films also have the problems that the film consisting of linear low density polyethylene lacks firmness during wrapping as a film for a shrink label and the films consisting of the above described compositions are inferior in low temperature shrinkage and retort ability.
In the field of twist wrapping, cellophane film has been used, but experiences a problem that it is inferior in moistureproofness and impact resistance at a low temperature such as in winter. On the other hand, monoaxially oriented polyethylene film has lately been used therefor, but this film tends to split. As this monoaxially oriented polyethylene film, there has been proposed a monoaxially oriented film consisting of a polyethylene having a density of 0.94 or more and a specified ratio of a melt index (MI.sub.10) measured at a load of 10 kg and a melt index (MI.sub.2.16) measured at a load of 2.16 kg, as disclosed in Japanese Patent Publication No. 794/1977 and Japanese Patent Laid-Open Publication No. 68212/1984.
In the prior art polyethylene films for twist wrapping, however, the clarity and twist maintenance are improved, but the split resistance is not sufficient. Splitting tends to occur, in particular, when aluminum foils are applied in the form of stripes or a square article is wrapped. Therefore, it has eagerly been desired to develop a film for twist wrapping, whereby the above described problems can be solved.